The present invention relates to an electromagnetic valve and, more particularly, to a pressurized medium regulating valve for an automatic transmission of a motor vehicle.
An electromagnetic valve is known comprising a magnetic housing, a electromagnetic coil received in the magnetic housing and a plurality of movable parts including a movable magnetic armature and a movable valve stem means for valve opening and closing.
Numerous embodiments of this type of electromagnetic valve are known and are, among other applications, used as a pressurized medium regulating valve in an automatic transmission of a motor vehicle. This type of pressurized medium regulating valve is described, for example, in German Published Patent Application 41 32 816. In order to damp the motions of the armature in this type of valve, its armature compartment must be already filled during assembly with a pressurized medium (oil). This type of hydraulic and/or viscous damping of the armature and/or the valve movable parts has however considerable disadvantages. A uniformly invariant filling of all valves during the manufacture of a number of electromagnetic valves in series with the pressurized medium can be guaranteed only by considerable effort and expense. Moreover a considerable number of steps are required for sealing the armature compartment. Since the gap height to a very large extent (to the third power) influences the damping in the hydraulic and/or viscous damping, already reduced manufacturing tolerances lead to a considerable variation in the valve characteristic curve. Furthermore hydraulically damped electromagnetic valves of this type are to a large extent sensitive to dirt and/or contamination, since dirty or contaminated pressurized medium can be forced into the armature compartment by a required volume or pressure balancing. Particularly that can lead easily to valve failure, because of contamination by ferromagnetic particles. The hydraulic and/or viscous damping effects the valve characteristic curve to a particularly large extent because of the large changes in the viscosity of the pressurized medium with temperature change. If the electromagnetic valve is used, for example as a pressurized medium regulating valve in an automatic transmission of a motor vehicle, the standard operating temperature range of the valve is from 230.degree. to 420.degree.K. The viscosity of the pressurized medium used for damping changes to a great extent in this temperature range. The viscosity of the pressurized medium can be changed in this range by as much as a factor of 3000. Because of that, it is nearly impossible to produce electromagnetic valves of this type with a nearly constant valve characteristic curve and uniform dynamics.